When an automobile is running at a high speed, collisions of small stones or the like against outside plates (strictly speaking, coated film surfaces) of the automobile are unavoidable. Accordingly, the collisions form cracks on the coated films or peel off the coated films from the substrates such as outside plates or other parts (so-called chipping is generated). If the chipping is generated on the coated film, water, etc., is permeated through the resulting flaw to form rust on the surface of the substrate (outside plate).
In particular, in such regions as the northern part of the U.S., Canada, and northern Europe, where rock salt and sand are scattered in large quantities on roads for the purpose of melting snow in the winter season, it is highly important for the coated films on outside plates of automotive body to have chipping resistance (i.e., impact resistance). Thus, it is desired to obtain a coated film which, even when small stones collide against it, does not suffer damages (chipping) and protects the substrate (outside plates of automotive body) against rusting.
The coating of outside plates of automotive body is generally conducted by a method in which a steel plate that has been chemical-treated with iron or zinc phosphate is coated successively with an electrodeposition coating paint (undercoating paint), an intermediate coating paint, and a topcoating paint. In order to improve chipping resistance and rust-preventive properties, various improvements have so far been proposed on electrodeposition coating paints (undercoating paints), intermediate coating paints, and topcoating paints and on coating methods therefor.
For example, JP-A-62-129184 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method for forming an anticorrosive coated film which comprises applying an electrodeposition coating paint on a steel plate, subsequently applying thereon an anticorrosive pigment-containing organic solvent-based coating paint (i.e., barrier coating paint) capable of forming a coated film having an elongation at break of 150% or higher and a stress at break of 20 kg/cm.sup.2 or higher, and then applying thereon an intermediate coating paint and a topcoating paint at a total thickness of at least 30 .mu.m on a dry basis, at least one of the intermediate coating paint or the topcoating paint being capable of forming a cured coated film having an elongation at break of 40% or lower and a stress at break of 300 kg/cm.sup.2 or higher. The barrier coat comprises a thermoplastic or thermosetting resin such as a thermosetting polyester resin composition, a thermoplastic polyurethane elastomer, a polybutadiene-containing cross-linking-curable resin composition, a styrene-butadiene copolymer, a vinyl acetate-ethylene copolymer, butyl rubber, or the like.
U.S. Pat. No. 4,985,500 (corresponding to JP-A-62-169869) discloses a primer composition consisting essentially of
(A) at least one resin having a glass transition temperature of not higher than -20.degree. C. and an elongation at break of not less than 400% at 20.degree. C., said resin being selected from the group consisting of polyolefin resin, polyacrylic resin, polyester resin, and modified resins thereof comprising linear high molecular weight elastomer; and PA1 (B) at least one cross-linking resin selected from the group consisting of melamine resin, urea resin, polyisocyanate, and blocked isocyanate; PA1 (B) an oil-free polyester resin which has been obtained by reacting a polybasic acid component other than a saturated alicyclic polybasic acid and tetrahydrophthalic acid with a polyhydric alcohol component containing 5 to 60 mol % of diethylene glycol and/or triethylene glycol and has an average molecular weight of 800 to 3,500 and a hydroxyl value of 70 to 140; and PA1 (C) an alkyl-etherified melamine resin having a weight-average molecular weight of 3,000 or less, PA1 (A) a modified polyisocyanate obtained from polycaprolactone-modified diol and an isocyanate compound in which the active isocyanate groups have been blocked and PA1 (B) a hydroxyl group-containing resin containing two or more hydroxyl groups per one molecule. PA1 (1) a coating process comprising electrodeposition coating, intermediate coating, and topcoating, to form at least one coated film selected from the group consisting of (a) to (e): PA1 (2) a coating process comprising electrodeposition coating and topcoating to form at least one coated film selected from the group consisting of (f) to (h): PA1 (A) a modified polyisocyanate obtained from polycaprolactone-modified diol and a isocyanate compound in which the active isocyanate groups have been blocked and PA1 (B) a hydroxyl group-containing resin containing two or more hydroxyl groups per one molecule. PA1 (i) a hydroxyl group-containing acrylic resin, PA1 (ii) a hydroxyl group-containing polyester resin, PA1 (iii) a diol of an aliphatic hydrocarbon having 2 to 8 carbon atoms, and PA1 (iv) a hydroxyl group-containing epoxy resin. Of these, a hydroxyl group-containing acrylic resin and a hydroxyl group-containing polyester resin are preferred. Particularly, a hydroxyl group-containing acrylic resin is preferred. PA1 (1) hydroxyl group-containing ethylenic monomers such as hydroxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, and N-methylolacrylamide; PA1 (2) carboxyl group-containing ethylenic monomers such as acrylic acid, methacrylic acid (MAA), crotonic acid, itaconic acid, fumaric acid, and maleic acid; and PA1 (3) alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl acrylate, isopropyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, lauryl (meth)acrylate, n-octyl acrylate, and n-dodecyl acrylate. PA1 (1) a coating process comprising electrodeposition coating, intermediate coating, and topcoating to form at least one coated film selected from the group consisting of (a) to (e): PA1 (2) a coating process comprising electrodeposition coating and topcoating to form at least one coated film selected from the group consisting of (f) to (h):
wherein a weight ratio as a solid resin content of component (A) to component (B) is 70-99:30-1.
That is, U.S. Pat. No. 4,985,500 discloses a primer composition obtained by a combination of a resin such as polyurethane resin, polyacrylic resin, polyvinyl resin, etc. and a hardener such as melamine resin, urea resin, polyisocyanate, etc.
JP-A-1-197569 discloses a coating composition which contains, as major components,
(A) a urethane polymer which has been obtained by reacting (i) a diisocyanate compound with (ii) a compound having 2 to 3 hydroxyl groups per one molecule on the average and has a number-average molecular weight of 5,000 to 50,000 and a hydroxyl value of 20 to 80;
the amounts of components (A) and (C) being from 20 to 50% by weight and from 20 to 40% by weight, respectively, based on the total solid amount of components (A), (B), and (C), with the remainder being constituted by component (B),
and which upon heating gives a cured coated film having a glass transition temperature of 10.degree. C. or lower and an elongation as measured at -20.degree. C. of 20% or higher.
Further, U.S. Pat. No. 4,888,244 (corresponding to JP-A-62-61675) discloses a method which comprises (i) forming a cationic electrodeposition coated film consisting mainly of a cationic resin having a functional group reactive to isocyanate group, and (ii) subsequently applying thereon an organic solvent-based coating paint which contains a polyisocyanate compound and forms a coated film having a static glass transition temperature of 0.degree. to -75.degree. C., followed by intermediate coating and topcoating. The preferred examples of the polyisocyanate compound which are enumerated in U.S. Pat. No. 4,888,244 (corresponding to JP-A-62-61675) include a product of reaction between hexamethylene diisocyanate and water, an adduct of xylylene diisocyanate with trimethylolpropane, an adduct of tolylene diisocyanate with hexamethylene diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, and lysine diisocyanate. In this prior-art method, such a polyisocyanate compound is used after being blocked with on ordinary blocking agent such as an oxime. As an example of the cationic resin having a functional group reactive to isocyanate group, a resin obtained by reacting an epoxy resin having a polyphenol with a cationic nature-imparting agent is given in the reference.
Furthermore, JP-B-61-36995 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method in which a solvent-based, one-pack type composition comprising a blocked isocyanate compound having a specific isocyanate group content and a polyhydroxy compound having a specific hydroxyl group content as major components is applied on an undercoated film to form a soft urethane resin film. In JP-B-61-36995, there is a description to the effect that as the blocked isocyanate compound in the above composition for forming a soft urethane resin film, use can be made of a compound produced by blocking, with an alcohol or the like, the active isocyanate groups of a polyisocyanate obtained by the addition reaction of (a) a polyisocyanate such as hexamethylene diisocyanate or the like (in an excess amount) with (b) a low-molecular polyol such as ethylene glycol, propylene glycol, 1,3-butylene glycol, neopentyl glycol, trimethylolpropane, or the like. In Examples given in the reference, a compound obtained by blocking hexamethylene diisocyanate with both methanol and 1,6-hexanediol or a compound obtained by blocking hexamethylene diisocyanate/biuret-type polyisocyanate with n-butanol alone is used as the blocked isocyanate compound.
However, the coating compositions and coating methods described above are still insufficient in giving a coated film having satisfactory chipping resistance. In addition, they still have problems concerning, for example, the efficiency of coating by the wet-on-wet method.
JP-A-63-43967 discloses a chipping-resistant coating paint containing, as major components, (a) an organic solvent-soluble urethane polymer which is an adduct of a diisocyanate compound with a polyol having 2 to 3 hydroxyl groups per one molecule on the average and has a number-average molecular weight of 10,000 to 100,000, (b) a hydroxyl group-containing resin having a specific number-average molecular weight, and (c) a blocked polyisocyanate compound. In JP-A-63-43967, polyester resins and polyethers are given as preferred examples of the hydroxyl group-containing resin as component (b). As examples of the blocked isocyanate compound (c), diisocyanates such as hexamethylene diisocyanate, isophorone diisocyanate, and tolylene diisocyanate and adducts of these diisocyanates with a polyol compound are given in the reference. As the polyol compound, polyether polyols and (hydroxyl group-containing) polyester resins are given therein. In Examples given in the reference, a compound obtained by blocking, with an oxime, an adduct of tolylene diisocyanate with both poly(tetramethylene glycol) and trimethylolpropane or an adduct of tolylene diisocyanate with trimethylolpropane is used as the blocked isocyanate compound (c).
However, since the coating paint of JP-A-63-43967 contains a resin component having a relatively high molecular weight (component (a) has a number-average molecular weight of 10,000 to 100,000), the coating paint gives a coated film having insufficient surface smoothness or it is necessary to use a large amount of solvent in order to attain surface smoothness. Use of a solvent in a large amount, however, is undesirable from the standpoints of environmental problem, economy, etc.